The overall objective of this application is to further our understanding of the role of carrier-mediated processes responsible for the uptake of xenobiotics from the systemic circulation into the liver -a critical first step in their hepatic elimination. A number of human hepatic basolateral membrane transporters have recently been cloned and functionally characterized with respect to the transport of endogenous compounds such as bile acids. We have cloned two novel human hepatic drug transporters with sequence homologies to the rat organic cation transporter (rOCT1), and the human organic anion transporter (hOATP). In addition, we have identified a mutation (Phe83- >Ser83)in the human hepatic sodium-dependent bile acid transporter, hNTCP, resulting from a single T->C substitution, that has a marked reduction in its transport activity. It is our hypothesis that these transport systems recognize a variety of drugs in clinical use, and that variability in function or activity has important implications in terms of drug elimination and therapy. Therefore, studies are proposed to test specific hypothesis through functional expression of individual transporters. The proposed studies in Specific Aim 1 will test the hypothesis that our newly cloned transporters, hOATP2 and hNLT, represent the biochemically characterized multispecific bile acid transporter, and a liver specific organic cation transporter, respectively. We will also define the transport kinetics of the mutant hNTCP transporter to assess whether this mutation alters substrate affinity. Additionally, the hypothesis will be tested that transport by hNTCP plays a pivotal role in the therapeutic response to tauroursodeoxycholic acid (in cholestatic liver disease) and to octreotide (in cancer chemotherapy). Specific Aim 2 focusses on site-directed mutagenesis and homologous chimera construction methods that will be used to better understand and identify key domains conferring transporter activity and substrate specificity. These studies will provide important insights regarding human hepatic drug transport systems, and enhance our understanding of the role of such processes in drug disposition.